Intelligent control of receiver linearity based on interference

ABSTRACT

A method and system for intelligently controlling the linearity of an RF receiver by selectively increasing the effective third-order intercept point (IP 3 ) value of a low noise amplifier (LNA)/mixer channel only when needed. A control signal is generated based on mode of operation (receive mode); received signal strength information (RSSI); transmit channel output power, as indicated by a Tx automatic gain control (AGC) signal; and the true received signal strength, as indicated by the pilot signal-to-noise ratio in a CDMA system. The control signal is then used to selectively increase the bias current—and thus linearity—of the LNA/mixer channel, or to select one of several LNAs having differing IP 3  values to effectively increase the linearity of the LNA/mixer channel.

TECHNICAL FIELD

This invention relates to electronic communication systems, and moreparticularly to intelligent control of linearity in electronicreceivers, such as code division multiple access (CDMA) receivers.

BACKGROUND

Certain types of radio frequency (RF) receivers that utilize a low noiseamplifier (LNA) need to perform under conditions that demand very highlinearity of the LNA and RF down conversion electronics. Such receiversare used in code division multiple access (CDMA) wireless communicationsystems. In a CDMA wireless communication system, users share all timeand frequency resources simultaneously. This is accomplished byassigning each user a distinct Walsh code or user-unique digital codewith special properties to achieve minimal interference between users.This code is added to the information data and modulated onto thecarrier. An identical code is used in the receiver which is used tocorrelate the two signals. The correlation process only passes data thatmatches the code sequence. In this way, non-valid signals (i.e., signalsfrom other users) are not decoded and appear as noise. Noise immunity isprovided by means of the code spreading the spectrum of the informationsignal to a much wider bandwidth than is needed for baseband signaltransmission. This ability of a CDMA system to reject unwanted signalsallows for operation in high noise environments. Further informationregarding CDMA systems is set forth in the well-known IS95 standard.

In one mode of operation, an IS95 compliant CDMA system permits duplexcommunications—that is, simultaneous transmission and reception of an RFsignal. One problem of this mode is that the presence of an interferencesignal as the transmit (Tx) channel approaches its maximum power outputgenerally causes cross-modulation of the Tx signal envelope. Thisresults in “in-band” interference in the Rx channel. Such in-bandinterference can cause degradation in the received signal quality andsubsequent call drop.

As is known in the art, the input third-order intercept point (IP3) ofan LNA/mixer channel can be adjusted to a high value to keepcross-modulation within tolerable limits. The IP3 is a virtualmeasurement of the signal strength at which the power of the 3rd-orderdistortion energy of a gain stage is as strong as the fundamental signalenergy. Using a high IP3 value increases the linearity of an LNA/mixerchannel. However, a high IP3 value also results in a high bias currentto the LNA/mixer channel, draining battery power and thus reducing bothtalk and standby time for an RF telephone. Nevertheless, maintaining ahigh IP3 value is the conventional approach to overcoming thecross-modulation problem noted above.

Accordingly, the inventor has determined that a better method and systemis needed to control cross-modulation during duplex communications of anRF communication system that overcomes the problems noted above. Thepresent invention provides such a method and system.

SUMMARY

The inventor has realized that, statistically, most of the operationtime of an LNA-based RF communication system, such as a CDMA cellulartelephone, is at low power levels of about −13 dBm. Thus, rather thanmaintain a high IP3 level at all times to manage cross-modulation duringthe relatively infrequent occasions where such cross-modulation occurs,the invention intelligently controls the linearity of an RF receiver byselectively increasing the effective IP3 value of an LNA/mixer channelonly when needed.

In particular, a control signal is generated based on mode of operation(power consuming); received signal strength information (RSSI); transmitchannel output power, as indicated by a Tx automatic gain control (AGC)signal; and the true received signal strength, as indicated by the pilotsignal-to-noise ratio. The control signal is then used to selectivelyincrease the bias current—and thus linearity—of the LNA/mixer channel,or to select one of several LNAs having differing IP3 values toeffectively increase the linearity of the LNA/mixer channel.

In one aspect, the invention includes a method and system forintelligently controlling the linearity of a receiver in a radiofrequency transceiver, including determining if the transceiver is in areceive mode, and if interference is present in the receiver, and iftransmit power within the transceiver is high, and if receiver powerwithin the transceiver is low; and controlling the effective third-orderintercept point (IP3) value of a LNA/mixer channel within the receiverin response to such determinations.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is block diagram of an RF transceiver in accordance with theinvention.

FIG. 2 is a flowchart showing the preferred decision logic for thepresent invention.

DETAILED DESCRIPTION

Circuit Configuration

FIG. 1 is block diagram of an RF transceiver in accordance with theinvention. An antenna 100 transmits and receives RF signals. A duplexer102 filters incoming (Rx) and outgoing (Tx) RF signals to block unwantedsignals and pass desired signals. The duplexer 102 also couples theantenna 100 to Tx signals from a transmitter 104 and couples Rx signalsfrom the antenna 100 to a receiver module 106. The receiver module 106includes an LNA/mixer channel, comprising a low noise amplifier 108 andmixer 110, for generating an IF signal from the Rx signal and a localoscillator signal, and an LNA/mixer bias control circuit 112, configuredin known fashion.

The output of the receiver module 106 is coupled to an intermediatefrequency (IF) circuit 113 which includes the necessary filters and gaincontrol amplifiers. The signal is further converted to I/Q (in-phase andquadrature) components to be processed by a conventionalanalog-to-digital (A/D) converter and envelope detector 114. The A/Dconverter and envelope detector 114 generates a received signal strengthinformation (RSSI) signal, which is proportional to the amplitude of thecomposite Rx signal before “despreading”. Presence of stronginterference signals can be (wrongly) indicated as a higher apparentreceived signal strength. The RSSI signal is generated by detecting theenvelope of the Rx signal after down conversion, and is relatively fastin response.

The output of the A/D converter and envelope detector 114 is coupled toa conventional CDMA processor 116 that preforms despreading by codecorrelation, CDMA pilot channel demodulation, and other known CDMAbaseband processing functions.

The CDMA processor 116 generates several outputs. One output is a Modesingle, indicating whether the transceiver is operating in duplex modeor simplex mode. A second output is a pilot signal-to-noise ratio,commonly known as Ec/Io, which is determined after despreading thereceived signal bandwidth. Interference energy is suppressed by theprocessing gain of the system. The Ec/Io signal is a measure of the truereceived signal strength. Comparing the RSSI signal and the Ec/Io signalcan be used to determine the presence of interference in the Rx channel.

The third output of the CDMA processor 116 is a Tx AGC signal that isnormally used to control the transmit channel output power. Thus, thevalue of the Tx AGC signal can be used to determine when Tx power isapproaching its maximum value in duplex mode.

The RSSI, Ec/Io, Tx AGC, and Mode signals are coupled to Decision Logic118, which generates a Linearity Control signal. In the illustratedembodiment, the Linearity Control signal is shown as controlling thecurrent output of the bias control circuit 112. In an alternativeembodiment, the Linearity Control signal can be used to select one ofseveral LNAs having differing IP3 values (e.g., using a switch ormultiplexor coupled to several LNAs). In either case, the LinearityControl signal selectively changes the effective IP3 value of theLNA/mixer channel of the receiver module 106 only when needed.

Decision Logic

The basic purpose of the Decision Logic 118 is to increase the effectivelinearity of the LNA 108 and mixer 110 when the transceiver is in aduplex mode or idle mode (when the receive circuitry must still beactive), and interference is detected, and Tx power is high, and Rxpower is low. FIG. 2 is a flowchart showing the preferred Decision Logic118 for the present invention.

The four signals noted above—RSSI, Ec/Io, Tx AGC, and Mode—are inputinto the Decision Logic 118 (STEP 200), and preferably temporarilystored (e.g., in registers or memory) (STEP 202). Of course, thesevalues will change over time, and the process shown in FIG. 2 normallywould be repeated periodically.

If the transceiver is not in a receive mode (i.e., a duplex mode such asa CDMA “Tx/Rx” mode, or an “Idle” mode) (STEP 204), then there is noneed to increase the linearity of the LNA/mixer channel (i.e., the LNA108 and mixer 110). Accordingly, the Linearity Control signal is set toselect a low IP3 LNA or decrease the LNA/mixer channel bias current(STEP 206), at which point the decision logic is done for the currentcycle (STEP 208).

If the transceiver is in a receive mode (STEP 204), then further testingis needed to determine if interference is present. In the preferredembodiment, interference is considered to be present if the RSSI valueis greater than the Ec/Io value plus a “false alarm” threshold value(STEP 210). The “false alarm” threshold value (generally determinedempirically) ensures that the difference between the RSSI value and theEc/Io value is significant. Accordingly, the RSSI and Ec/Io inputsignals are compared to determine if interference is present (STEP 212).If not, then the Linearity Control signal is set to select a low IP3 LNAor decrease the LNA/mixer channel bias current (STEP 206), at whichpoint the decision logic is done for the current cycle (STEP 208).

If interference is present (STEP 212), then an additional test isperformed to determine if the Tx power is high; in the illustratedembodiment, a threshold value of 20 dBm is used as a comparison value(STEP 214). If the Tx power is not currently high, then the LinearityControl signal is set to select a low IP3 LNA or decrease the LNA/mixerchannel bias current (STEP 206), at which point the decision logic isdone for the current cycle (STEP 208).

If the Tx power is currently high (STEP 214), then an additional test isperformed to determine if the Rx power is low; in the illustratedembodiment, a threshold value of −100 dBm is used as a comparison value(STEP 216). If the Rx power is currently low, then the Linearity Controlsignal is set to select a high IP3 LNA or increase the LNA/mixer channelbias current (STEP 218), at which point the decision logic is done forthe current cycle (STEP 220).

An additional test is performed to account for “test” conditions withinthe system in which two interference tones are applied at a −79 dBminput signal level. These interference tones can cause in-band spuriousfrequencies. However, the LNA gain is not important at this high signallevel. Accordingly, the Rx signal is tested to see if it is greater thanabout −85 dBm (this threshold value can be varied empirically). If not,then the Linearity Control signal is set to select a low IP3 LNA ordecrease the LNA/mixer channel bias current (STEP 206), at which pointthe decision logic is done for the current cycle (STEP 208). Otherwise,the LNA is bypassed or an attenuator (e.g., a PIN diode) is switched inbefore the LNA/mixer channel (STEP 224), at which point the decisionlogic is done for the current cycle (STEP 220).

Benefits of the invention include intelligently controlling thelinearity of an RF receiver by selectively increasing the effective IP3value of an LNA/mixer channel only when needed, and thus improves talktime during duplex communications. Further, when not in a receive mode(e.g., idle or duplex communication modes), power is conserved since ahigh IP3 value need not be maintained at all times in such modes (e.g.,sleep and FM modes). In one embodiment of the invention, it is estimatedthat the power saving in the receiver channel is greater than about 20%compared to the art. The invention also provides a method and system fordetermining the presence of interference on a received signal.

Computer Implementation

The Decision Logic 118 may be implemented in hardware or softwareexecuting on a general purpose processor, or a combination of both(e.g., programmable logic arrays). For example, the process described inFIG. 2 may be implemented in one or more computer programs executing onprogrammable systems each comprising at least one processor and at leastone data storage system (including volatile and non-volatile memoryand/or storage elements) for storing program code. The program code isexecuted on the processors to perform the functions described herein.Each such program may be implemented in any desired computer language(including machine, assembly, high level procedural, or object orientedprogramming languages) to communicate with a computer system. In anycase, the language may be a compiled or interpreted language. Each suchcomputer program is preferably stored on a storage media or device(e.g., ROM, CD-ROM, tape, or magnetic media) readable by a general orspecial purpose programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. The inventive system may alsobe considered to be implemented as a computer-readable storage medium,configured with a computer program, where the storage medium soconfigured causes a computer to operate in a specific and predefinedmanner to perform the functions described herein.

A number of embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, one of ordinary skill in the art will recognize that a numberof the steps shown in FIG. 2 are not sequence dependent, and thus may bedone in different orders. Accordingly, other embodiments are within thescope of the following claims.

What is claimed is:
 1. A method for intelligently controlling thelinearity of a receiver in a radio frequency transceiver, including thesteps of: (a) determining if the transceiver is in a receive mode, andif interference is present in the receiver, and if transmit power withinthe transceiver is high, and if receiver power within the transceiver islow; and (b) controlling the effective third-order intercept point (IP3)value of a low noise amplifier (LNA)/mixer channel within the receiverin response to such determinations.
 2. The method of claim 1, whereindetermining if interference is present in the receiver includes the stepof determining if a received signal strength information signalindicating the apparent strength of a signal received by the transceiveris significantly greater than the true received signal strength.
 3. Themethod of claim 2, further including the step of determining the truereceived signal strength as a pilot signal-to-noise ratio.
 4. The methodof claim 1, wherein determining if transmit power within the transceiveris high includes the step of comparing a transmit channel automatic gaincontrol signal within the transceiver to a selected threshold value. 5.The method of claim 1, wherein determining if receiver power within thetransceiver is low includes the step of comparing a received signalstrength information signal indicating the apparent strength of a signalreceived by the transceiver to a selected threshold value.
 6. The methodof claim 1, wherein determining if receiver power within the transceiveris low includes the step of comparing a pilot signal-to-noise ratiosignal indicating the true strength of a signal received by thetransceiver to a selected threshold value.
 7. The method of claim 1,wherein the transceiver is a code division multiple access transceiver.8. The method of claim 1, wherein controlling the effective IP3 value ofthe LNA/mixer channel includes increasing bias current to the LNA/mixerchannel.
 9. The method of claim 1, wherein controlling the effective IP3value of the LNA/mixer channel includes selecting one of several LNAshaving differing IP3 values.
 10. A method for intelligently controllingthe linearity of a receiver in a radio frequency transceiver, includingthe steps of: (a) determining: (1) if the transceiver is in a receivemode; and (2) if interference is present in the receiver by determiningif a received signal strength information signal indicating the apparentstrength of a signal received by the transceiver is significantlygreater than the true received signal strength; (3) if transmit powerwithin the transceiver is high by comparing a transmit channel automaticgain control signal within the transceiver to a selected thresholdvalue; (4) if receiver power within the transceiver is low by comparinga pilot signal-to-noise ratio signal to a selected threshold value; (b)controlling the effective third-order intercept point (IP3) value of alow noise amplifier (LNA)/mixer channel within the receiver in responseto such determinations.
 11. The method of claim 10, further includingthe step of determining the true received signal strength as a pilotsignal-to-noise ratio.
 12. The method of claim 10, wherein thetransceiver is a code division multiple access transceiver.
 13. Themethod of claim 10, wherein controlling the effective IP3 value of theLNA/mixer channel includes increasing bias current to the LNA/mixerchannel.
 14. The method of claim 10, wherein controlling the effectiveIP3 value of the LNA/mixer channel includes selecting one of severalLNAs having differing IP3 values.
 15. A radio frequency transceiver,including a receiver having an associated low noise amplifier(LNA)/mixer channel and a transmitter, and having intelligent control ofthe linearity of the LNA/mixer channel, including: (a) signal processingcircuitry for indicating a receive mode for the transceiver, theapparent power of a received signal, the true power of the receivedsignal, and the transmit power of the transceiver; (b) decision logic,coupled to the signal processing circuitry, for determining if thetransceiver is in a receive mode, and if interference is present in thereceiver based upon the apparent power of the received signal and thetrue power of the received signal, and if transmit power within thetransceiver is high, and if receiver power within the transceiver islow, and for generating a control signal in response to suchdeterminations; (c) means for controlling the effective third-orderintercept point value of the LNA/mixer channel in response to thecontrol signal.
 16. The radio frequency transceiver of claim 15, whereinthe decision logic determines if interference is present in the receiverby determining if the apparent power of the received signal issignificantly greater than the true power of the received signal. 17.The radio frequency transceiver of claim 15, wherein the signalprocessing circuitry includes an output indicating the apparent power ofa received signal as a received signal strength information signal. 18.The radio frequency transceiver of claim 17, wherein the decision logicdetermines if receiver power within the transceiver is low by comparingthe received signal strength information signal to a selected thresholdvalue.
 19. The radio frequency transceiver of claim 15, wherein thesignal processing circuitry includes an output indicating the true powerof a received signal as a pilot signal-to-noise ratio signal.
 20. Theradio frequency transceiver of claim 19, wherein the decision logicdetermines if receiver power within the transceiver is low by comparingthe pilot signal-to-noise ratio signal to a selected threshold value.21. The radio frequency transceiver of claim 15, wherein the signalprocessing circuitry includes an output indicating the transmit power ofthe transceiver as a transmitter channel automatic gain control signal.22. The radio frequency transceiver of claim 21, wherein the decisionlogic determines if transmit power within the transceiver is high bycomparing the transmitter channel automatic gain control signal to aselected threshold value.
 23. The radio frequency transceiver of claim15, wherein the transceiver is a code division multiple accesstransceiver.
 24. The radio frequency transceiver of claim 15, whereinthe means for controlling the effective third-order intercept pointvalue of the LNA/mixer channel in response to the control signalincludes a circuit for increasing bias current to the LNA/mixer channel.25. The radio frequency transceiver of claim 15, wherein the means forcontrolling the effective third-order intercept point value of theLNA/mixer channel in response to the control signal includes means forselecting one of several LNAs having differing IP3 values.
 26. A radiofrequency transceiver, including a receiver having an associated lownoise amplifier (LNA)/mixer channel and a transmitter, and havingintelligent control of the linearity of the LNA/mixer channel,including: (a) signal processing circuitry for indicating a receive modefor the transceiver, the apparent power of a received signal as areceived signal strength information signal, the true received signalstrength of the received signal as a pilot signal-to-noise ratio signal,and the transmit power of the transceiver as a transmit channelautomatic gain control signal; (b) decision logic, coupled to the signalprocessing circuitry, for generating a control signal in response todetermining: (1) if the transceiver is in a receive mode; and (2) ifinterference is present in the receiver by determining if a receivedsignal strength information signal indicating the apparent strength of asignal received by the transceiver is significantly greater than thetrue received signal strength; (3) if transmit power within thetransceiver is high by comparing the transmit channel automatic gaincontrol signal to a selected threshold value; (4) if receiver powerwithin the transceiver is low by comparing the pilot signal-to-noiseratio signal to a selected threshold value; (c) means for controllingthe effective third-order intercept point value of the LNA/mixer channelin response to the control signal.
 27. The radio frequency transceiverof claim 26, wherein the transceiver is a code division multiple accesstransceiver.
 28. The radio frequency transceiver of claim 26, whereinthe means for controlling the effective third-order intercept pointvalue of the LNA/mixer channel in response to the control signalincludes a circuit for increasing bias current to the LNA/mixer channel.29. The radio frequency transceiver of claim 26, wherein the means forcontrolling the effective third-order intercept point value of theLNA/mixer channel in response to the control signal includes means forselecting one of several LNAs having differing IP3 values.
 30. Acomputer program, residing on a computer-readable medium, forintelligently controlling the linearity of a low noise amplifier(LNA)/mixer channel within a receiver in a radio frequency transceiver,the computer program comprising instructions for causing a processorwithin the transceiver to: (a) determine if the transceiver is in aduplex communication mode, and if interference is present in thereceiver, and if transmit power within the transceiver is high, and ifreceiver power within the transceiver is low; (b) control the effectivethird-order intercept point (IP3) value of the LNA/mixer channel inresponse to such determination.